Nearsightedness (Myopia)

Myopia is the medical term for nearsightedness. Myopia occurs when an eye is too long or the cornea's curvature is too steep. Light rays entering the eye do not come to a sharp focus on the retina. Instead, they focus farther forward, producing a blurred image. The term nearsighted means that myopic individuals can see "near" objects clearly without glasses, but objects further in the distance are blurred. The more myopic, the more blurred distant objects appear, the higher your eyeglass prescription and the thicker your glasses needed for correction. 

Myopia can be corrected by any method that reduces the total refractive power of the eye. Eyeglasses and contact lenses do this by putting in front of the eye "negative" lenses that are thicker at the edge than in the center. LASIK procedure by flattening the central part of the cornea, decreases the refractive power of the eye.

Farsightedness (Hyperopia)
Hyperopia is the medical term for farsightedness. This occurs when an eye is too short or the corneal curvature is flat . Light rays entering the eye do not come to a sharp focus on the retina. Instead, they focus farther backward, producing a blurred image. Farsighted eyes are too short and need a "plus" power to help them focus.

Astigmatism

Astigmatism is the most common refractive (focusing) error. Astigmatism creates distorted and ghosting vision. It occurs when the front of the cornea is somewhat oval-shaped rather than round .The two different curves in such a corneal surface each bend light rays to a separate focus point.

Astigmatism correction makes all the rays of light focus at the same distance so that they all fall correctly on the retina. Small degrees of astigmatism do not impact vision significantly, but with larger amounts, distortion and blur occur. Astigmatism can occur alone as the sole optical error, or may occur together with either myopia or hyperopia.